Silicon products are used in various applications. In some applications, it is desirable to produce silicon products with a high purity, e.g., purity exceeding metallurgical grade silicon. For example, production of high-density integrated circuits requires wafers of monocrystalline silicon of high purity. Metal impurities on the silicon product, such as copper, gold, iron, cobalt, nickel, chromium, tantalum, zinc, tungsten, titanium, magnesium, molybdenum, and aluminum can be harmful to the production of such integrated circuits.
In an effort to produce silicon products with high purity, contact of the silicon product with other materials is generally avoided to prevent contamination of the silicon product. For example, contact of the silicon product with metal containing materials is avoided to prevent the metal from being transferred to the silicon product thereby contaminating the silicon product. Metal contamination of the silicon product can severely limit the end use of the silicon product. As such, the use of metal containing material in contact with the silicon product is avoided.
To reduce the exposure of the silicon product to metal containing materials, items contacting the silicon product can be made from or coated with plastic materials or other materials not dissolved by silicon etchants. For example, when the silicon product is polycrystalline silicon, hammers made of a plastic material can be used to chunk the polycrystalline silicon for further processing of the polycrystalline silicon. However, contact of the plastic or other material not dissolved by silicon etchants with the silicon product can result in a transfer of the plastic or other material not dissolved by silicon etchants to a surface of the silicon product. The plastic or other material not dissolved by silicon etchants on the surface of the silicon product is a source of carbon contamination, which can limit the end use of the silicon product.
In most cases, the contamination from the plastic or other material not dissolved by silicon etchants on the silicon product is insignificant and does not limit subsequent processing of the silicon product. However, in some applications, such as high performance electronic applications the contamination from the plastic or other material not dissolved by silicon etchants on the surface of the silicon product, such as polycrystalline silicon, is a concern and needs to be quantified. Current test methods do not have adequate sensitivity for these high purity applications. For example some methods do not concentrate the plastics or use a large enough sample size. Variability in any process results in differing amounts of plastic or other surface material to be present on different pieces of silicon. Because this amount can be highly variable, testing only one or a few chunks is unlikely to give a useful test (with results ranging from very low to very high numbers). As with any statistical sampling, testing a larger amount of material reduces average variation, so it is beneficial to test a sample larger than a single piece. The test described here is flexible in that it can be used to test a significantly larger sample of silicon. This is in contrast to tests which only test a single piece or are otherwise limited in their sample size. Vessel float zoning methods do not differentiate surface carbon contamination and bulk carbon contamination. Standard Fourier Transform Infrared (FTIR) spectroscopy methods commonly used in the polysilicon industry only test the bulk carbon. Therefore, there is a need to determine the contamination from the plastic, or other material not dissolved by silicon etchants on the silicon product as no test method to quantify the amount of plastic, or other material not dissolved by silicon etchants and elementally classify the plastics, or other material not dissolved by silicon etchants on the silicon product existed prior to the method disclosed herein.